in the ecosystems. Similarly, the study of the most important physiologi- 

 cal and biochemical parameters of various aquatic organisms dwelling in a 

 polluted water-body and the detection of essential disturbances in func- 

 tioning of living systems do not reveal harmless concentrations of chemi- 

 cal agents. Therefore, the main task of the biological setting of stan- 

 dards, i.e., establishment of the MPC, may be solved only under experi- 

 mental conditions on the most sensitive test-objects or representative or- 

 ganisms of any natural ecosystem. Of course, such a scheme of experiments, 

 i.e., the separate use of the most sensitive elements of the ecosystems, 

 leads to certain simplifications of the real situation in the water-body. 

 It would be, however, a naive assumption to expect that these simplifica- 

 tions might be avoided by experimenting in ponds or canals rather than in 

 aquaria, or complex "natural ecosystem" as opposed to individually sensi- 

 tive test objects. This delusion comes from confusing the main tasks of 

 the principles and methods of sanitary hydrobiology and aquatic toxicology. 

 One should not be embarrassed by the experimentally unavoidable "simplifi- 

 cations" of a real situation, just as medical science is not embarrassed 

 when sanitary-hygienic standards for chemical substance are established. 

 The sanitary-hygienic MPCs are meant to secure the safety of human beings, 

 but they are established in experiments using small rodents or other 

 larger mammals (rabbit, dog). 



Similarly, the genetic investigations designed to determine the MPC 

 will most likely be performed on dAo6opkila t>p., a classical test animal 

 in genetic investigations, having a number of advantages over laboratory 

 mammals. In this regard, the position of aquatic toxicologists is easier 

 since the possibility of studying the MPC directly exists. This direct 

 application also allows the selection of the most sensitive species. The 

 use of the most sensitive and least resistance components of natural 

 ecosystems, namely fish and aquatic invertebrates, makes the experimenting 

 on more complex ecosystems for the establishment of the MPC unnecessary. 



The concept of "natural ecosystems" itself has no single meaning, and 

 will be essentially different for each experimental water body, to say 

 nothing of those natural waters which serve as "receivers" of waste 

 waters. Figuratively speaking, "the natural ecosystem" of the experi- 

 mental small pond differs from "the natural ecosystem" of a reservoir or a 

 lake to much greater extent than do small laboratory rodents from man. 

 Consequently the appeals to change the experiments on test-objects in 

 aquaria for experiments on "natural ecosystems" in small ponds are lacking 

 serious scientific foundations, and do not take into consideration the 

 needs of today's life, i.e., to establish in the shortest time the MTCs of 

 hundreds of substances entering waters in connection with the appearance 

 of new branches of industry, modernization of technological processes and 

 the advances of chemistry in agriculture. 



It is well known that the metazoans, which constitute the basis of 

 grazer circulation, are more sensitive to toxicants than are the unicell- 

 ular organisms. Among the metazoans, the vertebrates are more sensitive 

 to various toxicants, specifically, organic compounds, than are the 

 invertebrate forms. This fact served as a basis for wide use of various 

 species of fish when establishing the MPC values with the help of the 



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